Spinal Muscular Atrophy (SMA)

Spinal muscular atrophy (SMA) is a genetic disease with an assortment of clinical manifestations and a wide range of clinical severity.1 While some patients with the disease start to show symptoms even before birth (as in the case of SMA type 0, the most severe form of the disease), others don’t show any signs of disease until well into adulthood (as in the case of SMA type 4).

Symptom Onset and SMA Types

There are 5 types of SMA. Classification of SMA is based on age at onset of symptoms and maximum motor function achieved.1 Accordingly, in SMA type 0, symptoms appear at birth, but the first signs are often noticeable prenatally. In SMA type 1, early signs appear by age 6 months, while in SMA type 2 they become apparent between ages 6 and 12 months. In SMA type 3, symptoms may appear any time between 18 months of age and adulthood. Finally, in SMA type 4, symptoms usually appear after age 30.

First Signs of SMA Types

Patients with SMA type 0 may present with reduced fetal movements. They have severe hypotonia and joint abnormalities at birth, as well as dysphagia and respiratory difficulties.2

The early signs of SMA type 1 consist of hypotonia and poor head control. Suckling and swallowing difficulties may also be noticeable.3

Babies with SMA type 2 can sit independently but have noticeable muscle weakness in the extremities.4

Patients affected by SMA type 3 may experience frequent falls and have difficulty getting up from a seated position and going up stairs.5

Those with SMA type 4 may experience weakness in their leg muscles, but this usually does not affect their ability to walk.6

Studying Early Signs of Neuromuscular Diseases

In 2010, the National Task Force for Early Identification of Childhood Neuromuscular Disorders conducted an interview project funded by the Centers for Disease Control and Prevention (CDC). The team interviewed more than 1000 parents of children, aged 6 months to 9 years, whose concerns were later identified as a neuromuscular disorder.

According to a published report on the results, the first signs of neuromuscular conditions, including SMA, were poor head control, hypotonia, limited movement, tremors, regression, delayed progression of motor skills, issues with rising from the floor, standing, walking, and climbing stairs, and general weakness.7

Parents expressed their concerns in each area using statements including “His head flops down,” “She feels like a rag doll,” “He is not moving much,” and “His tongue trembles.” Caregivers of children with SMA expressed concerns such as “She can’t hold her head up and can’t do ‘tummy time,’ ” “He was slow in crawling and won’t put any weight on his legs,” and “He has hand tremors, is falling down a lot, and is walking funny.”

Some of these signs and symptoms may be caused by less severe conditions and resolve by themselves, but some may be caused by genetic diseases such as SMA. If a child has suddenly developed regression, seems stiff, rarely moves, appears floppy and loose in the limbs, and their lower jaw and tongue trembles chronically, they should be assessed and tested for a neuromuscular condition, including SMA.8

Receiving a diagnosis of a progressive genetic condition such as SMA can come as a shock to most families. Adult patients and their families should be offered counseling and psychological support at the time of diagnosis and as the disease progresses.9

Reviewed by Michael Sapko, MD on 7/1/2021


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  3. Type 1. SMA Europe. Accessed June 4, 2021.
  4. Spinal muscular atrophy type 2. Genetic and Rare Diseases Information Center. Accessed June 4, 2021.
  5. Spinal muscular atrophy type 3. Genetic and Rare Diseases Information Center. Accessed June 4, 2021.
  6. Spinal muscular atrophy type 4. Spinal Muscular Atrophy Support UK. Accessed June 4, 2021.
  7. Signs of weakness by parent report. Child Muscle Weakness. Accessed June 4, 2021.
  8. Early signs of spinal muscular atrophy. Cure SMA. Accessed June 4, 2021.
  9. Qian Y, McGraw S, Henne J, Jarecki J, Hobby K, Yeh WS. Understanding the experiences and needs of individuals with spinal muscular atrophy and their parents: a qualitative study. BMC Neurol. 2015;15:217. doi:10.1186/s12883-015-0473-3 
  10. What is 5q spinal muscular atrophy? Spinal Muscular Atrophy UK. Accessed June 4, 2021.
  11. Butchbach MER. Copy number variations in the survival motor neuron genes: implications for spinal muscular atrophy and other neurodegenerative diseases. Front Mol Biosci. 2016;3:7. doi:10.3389/fmolb.2016.00007
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